This invention relates generally to casting and more specifically metal casting using ceramic containing molds.
A substantial number of metal castings are created by pouring molten metal into a ceramic mold. In sand casting, the mold is typically made of sand, held together with various binders. In investment casting, the mold is typically made of refractories, such as alumina powder, bound together by silica. A significant problem relates to a discrepancy in the change in the dimensions of the ceramic mold and the forming metal within the mold, as the casting cools.
The problem has two aspects. First, as most metals solidify from liquid, there is a significant volume change, generally a shrinkage, on the order of 5%, by volume. Second, the coefficient of thermal expansion of metals is typically substantially higher than that of ceramics. Thus, as the solidified casting continues to cool, the metal will shrink more than the ceramic.
Both these effects can lead to rupture or distortion of the casting. As the casting freezes, it is quite weak, and at that stage, can actually be torn apart by the constraints imposed by the ceramic mold. Such a failure is called a xe2x80x9chot tear,xe2x80x9d or xe2x80x9chot crack.xe2x80x9d As the casting continues to cool, it can be distorted by the constraints imposed by the ceramic mold.
Significant effort is expended in the casting art toward minimizing the difficulties imposed by the differential in change in dimension. Such efforts include control over the melt feeding pattern and freezing pattern through the use of risers and chills. In investment casting, the composition of the shell may be tailored, to promote its breakage during the cooling phase of the casting operation.
Typically a mold is made by providing a pattern whose outside shape is the shape of the object to be made. The pattern is made of a material that will burn away at a later stage. This pattern is dipped into a ceramic slurry, which forms a coating thereon. The coated shell is subsequently dipped in a different ceramic slurry, with different properties from the first ceramic slurry. The coated shell is dipped again and again into a succession of slurries, each being different from the previous slurry and the pattern is removed. The result is a hollow body coated with a succession of different coatings, much like layers of onion skin (except that the center of the body is hollow). The designer chooses the different coatings with the hope that they will themselves rupture, as the casting cools, thereby preventing the casting from distorting. Additional customization to promote or prevent mold rupture at specific locations can be provided by causing the shell to be thicker in certain locations.
Typically, the innermost layer is composed of a relatively fine grained ceramic slurry. Each subsequently applied slurry will typically be composed of coarser ceramic particles, such that each layer is successively coarser than the previous. The resultant body is porous. The distribution of particles and particle free void regions is uniform in each layer, and random within that layer, depending on the size and shape and uniformity of particles in the slurry, as well as the liquid content, both quantitative and qualitative. The voids are approximately the same size as the particles, and typically particles of any given layer can not pass through the voids in that layer. Further, the designer can not specify where in any layer, relative to the location of features in the casting, particles or voids will reside. Further, the designer has limited, if any, control over how, or where in the ceramic slurry coatings, relative to the features in the casting, the mold will break. The pattern of solid particles and voids within a layer is not predetermined or controllable, or repeatable in any way.
Typically if the mold is made, and it breaks prematurely, or in the wrong place, the designer will change the next attempt by using different coatings, or by increasing the thickness in different locations.
The following illustrates the problem quantitatively.
As the molten metal solidifies, it contracts. It is important to note that this contraction results from the phase change, not a temperature change, which is discussed below. For example, aluminum contracts by between 3.5 and 8.5%, by volume (or equivalently, between about 1.2 and 3.8% in linear dimension) upon solidification. Nickel alloys contract approximately 3% by volume upon solidification. (ASM Handbook, Volume 15, p. 768 (aluminum) and p. 822 (nickel).)
In an extreme case, a casting may contract by this full amount upon solidification. More typically, a mold is designed with xe2x80x9crisers,xe2x80x9d which continue to feed molten metal to the casting as it cools, to minimize the impact of the solidification shrinkage. However, in castings of complex geometry, such approaches often do not fully compensate for the shrinkage and some dimension change of the casting results.
As the casting continues to cool, after solidification, it contracts further. For example, the coefficient of thermal expansion of aluminum alloys and nickel alloys are approximately 20xc3x9710xe2x88x926/xc2x0C. and 14xc3x9710xe2x88x926/xc2x0C., respectively. Ceramic mold materials contract far less. For example, in the range of 200-1200xc2x0 C., the coefficient of thermal expansion of alumina is approximately 7xc3x9710xe2x88x926/xc2x0C. and that of fused silica is approximately 1xc3x9710xe2x88x926/xc2x0C. Thus, for example, when an aluminum casting cools in a ceramic mold made of alumina from a solidification temperature of 630xc2x0C. to room temperature, the linear shrinkage of the metal is approximately 1.25% (after solidification) but the ceramic only shrinks approximately 0.4%. Thus, for example, a casting of 10 inch dimension (25.4 cm) will shrink 0.09 in. (0.22 cm) more than the mold that is containing it.
Thus, the difference in the degree of shrinkage of the casting upon solidification and the shrinkage of the casting after solidification due to cooling on the one hand, at the same time as the degree of shrinkage in the the mold on the other hand may cause ruptures or distortions in the casting or mold rupture.
Such failure of the mold is uncontrolled and usually harmful. For instance, it may occur while the molding material is still liquid, or flowable, thereby resulting in leakage of the molding material from the mold. Or, it may occur at a location that does not provide the stress relief to the casting that is required.
In general, the present invention solves the problems that arise from the differential changes in geometry inherent to casting metal in a ceramic mold, by control of the internal morphology. By xe2x80x9cinternal morphologyxe2x80x9d it is meant, between the surfaces of the mold that face the casting, and that face the external environment. Specifically, layered fabrication techniques are used to create a ceramic mold. Control may be exercised, not just over the geometry of the inner and outer walls of the mold themselves, but also of the morphology of the structure between the walls. For example, an internal geometry composed of a cellular arrangement of skeletal elements and voids may be created within the mold wall. Through such control of the internal morphology, structures may be designed and fabricated so that the ceramic mold is virtually guaranteed to fail at an appropriate time during the solidification and/or cooling of the casting. Thus, the casting itself is not damaged. As used herein in the context of the mold, xe2x80x9cto failxe2x80x9d means to break, rupture or bend past an elastic limit. If a structure xe2x80x9cfailsxe2x80x9d under a loading condition, it will not return to its original form after the loads are removed. The goal of the present invention is to design and control the mold to fail (break, rupture or bend past an elastic limit) and thus to avoid rupture, or even distortion, of the casting.
Such collapsing molds of the invention typically consist of a thin layer of ceramic, which defines the casting cavity. This layer must be thin enough to fail due to the stresses induced (primarily compressive) by the metal next to it and partly adherent to it. Such a thin layer, however, would not be strong enough to be manipulated, handled or to enable transport of the mold before use, or to withstand the pressure and forces of the molten metal during pouring of the casting. Thus another aspect of the present invention is to include a support structure with a morphology that supports the thin wall that defines the casting geometry, yet that also fails as the casting solidifies and/or cools.
At least the following two failure mechanisms may be exploited in the design of the support structure: failure by bending in the structure; and failure under compressive loads, either by buckling of a support member or, more likely by the breakage of the member under the compressive loading. The breakage of the support structure may also be due to a combination of bending and compression.
Thus, the designer has total control over where in the mold, relative to the locations of features in the casting, skeletal elements are, and also how they are oriented. Thus, the designer has total control over where, relative to the locations of features in the casting, the mold will be more likely to fail, and thus to avoid damage to the casting at even the most delicate of features.
A preferred embodiment of the invention is a mold for casting a part made from a molding material, such as metal, which material experiences dimensional change during a mold process. The mold comprises a thin inner shell, which defines a three dimensional cavity that will establish locations of features of the casting part, is impervious to flow of liquid molding material therethrough, and is configured to not fail under any stresses arising within the inner shell due to pouring of the liquid molding material into the cavity. The shell is, however, configured to fail under stresses arising within the inner shell as any such molding material solidifies or cools. The mold also includes a three-dimensional support body that supports the inner shell. The support body is defined by an internal structure of supporting skeletal elements in predetermined locations and orientations relative to features of the casting part, and voids between the skeletal elements. Like the shell, it is configured to not fail under any stresses arising within the support body due to pouring of the liquid molding material into the cavity; but it is configured through the locations and orientations of the skeletal elements within its internal structures to fail at predetermined regions relative to the locations of features of the casting part, under stresses arising within the support body as any such molding material solidifies and cools. The stresses within the support body and the shell that arise during a molding process are due to at least one phenomena of: any dimensional change of any molding material residing in the cavity upon solidification; and a difference between the coefficients of thermal expansion of the support body on the one hand and any such molding material on the other hand.
The support body is typically contiguous with the inner shell. The skeletal elements are arranged such that during any dimensional changes of the molding material, enough of the skeletal elements would fail to prevent distortions to the part being cast. Failure may be by bending, compression, or buckling.
Typically, the support body is a cellular body, which may have rectilinear cells, either equal or unequal in size. There is also typically a continuous open path from within each cell to outside the body.
In a preferred embodiment, the support is composed of ceramic powder particles that have been joined together. The voids in the support body typically have a linear dimension that exceeds three times the average linear dimension of the powder particles.
The support body may comprise photocurable polymer loaded with ceramic particles.
Frequently, the skeletal elements and voids are arranged in at least one story, which story comprises a course connected to struts, which are connected to the thin inner shell. The support structure typically comprises a plurality of stories, each of which comprises a course, connected to struts, which are connected to an adjacent story. The skeletal elements may be struts, lattice elements (which are similar) or sheets, or any combination thereof. The mold may include an outer shell that contacts an outermost story. The inner shell may have an open boundary with which the support body is not contiguous, thereby forming a tub-like mold.
In a preferred embodiment of this aspect of the invention, in a specified region, the struts have a cross-sectional area of b2, and are spaced from adjacent struts a distance w. They are sized and spaced such that the ratio b/w is greater than the square root of the ratio of the hydrostatic pouring pressure of the molding material, over the minimum compressive strength of the material from which the struts are made. Such a mold will not fail under the charging of the mold with mold material.
Further, in another related preferred embodiment, adjacent a specified region of the casting, where the casting has a feature having a linear dimension D, the struts have a cross-sectional area of b2 and each strut has a length L, and a neutral axis located a distance h from a surface of the strut which is under maximum tensile load. The struts are made from a material having an elastic modulus E, and the relative strain between the casting and the thin shell is xcex5R. The struts are sized and shaped further such that the maximum tensile breaking strength of the struts is less than             6      ⁢      Eh              L      2        *                              ϵ          R                ⁢        D            2        .  
This will ensure that the struts do break before the casting is damaged. For square struts, the maximum tensile breaking strength of the struts is less than             3      ⁢      Eb              L      2        *                              ϵ          R                ⁢        D            2        .  
An alternative version of a preferred embodiment of the invention is a mold comprising a thin inner shell that defines a three dimensional cavity that will establish locations of features of the casting part. The mold also includes a three-dimensional foraminous support body that is contiguous with and substantially surrounds the inner shell. The support body is constructed of skeletal elements with voids therebetween, the voids comprising a network having an open pathway from each void to outside the support body and the skeletal elements being positioned and oriented according to a designed morphology at predetermined regions relative to the locations of features of the casting part.
Many of the features of the embodiments discussed above are also aspects of this embodiment of the mold of the invention. The skeletal elements may be ceramic. The skeletal elements may be arranged in cells, as discussed above. The struts and sheets of the support body may have a cross-section that varies along their length and may be straight or curved. Furthermore, the skeletal elements and voids may be arranged to provide a predetermined pattern of heat transfer from any casting material as it cools to form a part to be made with the mold, which heat transfer varies at different locations in the pattern.
Yet another preferred embodiment of the invention is a method of making a mold using a solid free form layered fabrication technique. The method comprises the steps of: providing a machine readable model of a mold geometry comprising a thin shell and a support body, in accordance with any of the embodiments described above, and using the model to drive a solid free form layered fabrication machine, building up, by layers, a mold that is defined by the geometry.
The step of building up, by layers, may comprise the steps of depositing a layer of a powder material in a confined region and then applying a further material to one or more selected regions of the layer of powder material which will cause the layer of powder material to become bonded at the one or more selected regions that will become the inner shell and a plurality of skeletal elements of the support body. The steps of depositing powder and applying further material are repeated a selected number of times to produce a selected number of successive layers. The further material causes the successive layers to become bonded to each other to form the inner shell and the skeletal elements of the support body. The method also includes the step of removing unbonded powder material which is not at the one or more selected regions, to form the cavity and voids between the skeletal elements. The powder is removed through passageways that exist between the voids and the outside of the mold. Molding material is introduced to the inner shell through a passage provided for this purpose.
According to one aspect of this embodiment of the invention, the model of the mold comprises a rectilinear cellular body defined by substantially parallel planar stories. The repeated step of depositing a layer of powder material may comprise depositing a layer in a plane that is either substantially parallel to or oblique to the substantially parallel stories.
A related aspect of such an embodiment includes providing a model of skeletal elements arranged to provide a predetermined pattern of heat transfer from any casting material as it cools to form a part to be made with the mold, which heat transfer varies at different locations in the pattern.
In accordance with various preferred embodiments of the invention, the free form layered fabrication technique may be any of: Three Dimensional Printing, Selective Laser Sintering, Stereo Lithography, CAM-LEM, Fused Deposition Modeling and Ballistic Particle Manufacturing.
The method of making a mold also includes providing a model having skeletal elements sized, shaped and spaced as described above, to ensure that the mold will support the molding material as it is poured into the cavity, and so that it will also fail as the molding material solidifies and cools, to avoid its damage.
Still another preferred embodiment of the invention is a method of molding a part. The method comprises the steps of providing a mold that has an inner shell and a support body, according to any of the embodiments described above, made according to any of the methods described above. The method further entails providing liquid molding material in the cavity and maintaining the mold under conditions such that the molding material solidifies into the part; and such that the solidified part and the mold cools. The method further requires maintaining the shell and supporting body such that both fail at predetermined regions relative to the locations of features of the casting part, as the molding material experiences dimensional change, and such that the molding material deforms less than its yield strain as it solidifies and cools. Finally, the failed mold is removed from the solidified, cooled part.
In a preferred embodiment, the method of providing the mold is a method of providing layers of powdered material, as described above. Typically, the powdered material is a ceramic material, and the molding material is a metal.
The mold will typically fail before any features of the casting if certain requirements are met. In a specified region of the support body, adjacent where a feature of the casting has a linear dimension D, the skeletal elements comprise struts, each strut having a length L, a cross-sectional area of b2, and a neutral axis located a distance h from a surface of the strut which is under maximum tensile load. If the strut is made from a material having an elastic modulus E, and if they are spaced from adjacent struts a distance w, and if the relative strain between the casting material and the thin shell is xcex5R, then the struts should be sized and spaced such that the tensile breaking strength of the struts is less than                     6        ⁢        Eh                    L        2              *                            ϵ          R                ⁢        D            2        ,
to ensure failure. For square struts, this reduces to the maximum tensile breaking strength of the struts being less than             3      ⁢      Eb              L      2        *                              ϵ          R                ⁢        D            2        .  
Yet another embodiment of the invention is a mold comprising a thin inner shell that defines a three dimensional cavity; and a three-dimensional foraminous support body that is contiguous with and substantially surrounds the inner shell. The support body comprises skeletal elements with voids therebetween, the voids comprising a network having an open pathway from each void to outside the support body, the skeletal elements and voids arranged to provide a predetermined pattern of heat transfer from any casting material as it cools to form a part to be made with the mold, which heat transfer varies at different locations in the pattern. The skeletal elements may be struts, lattice elements, sheets, or any combination thereof. They may be arranged to impede the transfer of heat from the casting material or to direct the transfer of heat from the casting material along a predetermined path. The sheets may be arranged as radiation or convection shields. The thermal control mold of the invention may also include any of the other geometrical features discussed above.
Still another preferred embodiment of the invention a method of making a mold, using a solid free form layered fabrication technique. The method comprises the steps of providing a machine readable model of a mold geometry as described immediately above, with the heat control properties enumerated.